The present invention relates to a process for the preparation of coated peroxygen compounds, in particular coated sodium percarbonate with an increased active oxygen stability.
Many peroxygen compounds, also called active oxygen compounds, such as sodium percarbonate (2 Na.sub.2 CO.sub.3.3H.sub.2 O.sub.2) and sodium perborate monohydrate and tetrahydrate, are used as the active oxygen component in detergents, bleaching compositions and cleaning compositions. Because of the inadequate storage stability of sodium percarbonate in a warm-humid environment and in the presence of various components of detergents and cleaning compositions, sodium percarbonate must be stabilized against the loss of active oxygen (Oa). An essential principle of the stabilization consists of surrounding the sodium percarbonate particles with a coating of components having a stabilizing action.
Various substances and substance combinations which are applied in one or more layers to a core of sodium percarbonate have been proposed for stabilizing sodium percarbonate. Examples of coating components are: hydrate-forming inorganic compounds, such as sodium sulfate, magnesium sulfate, alkali metal carbonates and bicarbonates, and furthermore alkali metal silicates, alkali metal and alkaline earth metal borates and alkali metal and magnesium salts of organic carboxylic acids, such as fatty acids and citric acid. Coating of sodium percarbonate and other peroxygen compounds is at present chiefly carried out in fluidized bed processes, wherein a solution of the coating components is applied to seeds in a fluidized bed and water is evaporated off. Reference is made to DE-OS 24 17 572, German Patent 26 22 610 (corresponds to U.S. Pat. No. 4,105,827), WO 95/02555 and WO 97/19890 by way of example.
In the known fluidized bed coating of sodium percarbonate, 0.5 to 25 wt.% of coating components is applied. Although the stability increases as the amount of coating increases, the active oxygen content also decreases. There is accordingly an interest in minimizing the content of coating components in coated sodium percarbonate, while maintaining a good stability in detergents. While fluidized bed coating has proved to be superior to other coating processes, for example coating in a mixer with subsequent drying, the inventors of the present application have demonstrated by dyeing experiments that coating using a fluidized bed reactor with a conventional inflow base, for example a perforated or sieve base or so-called Conidurboden.RTM., is incomplete both in a discontinuous procedure and quite so in a continuous procedure. This deficiency, which manifests itself in an inadequate stability of the active oxygen compound, can be overcome only by applying a thicker coating layer, although at the expense of a low active oxygen content.
An object of the present invention was accordingly to coat active oxygen compounds, in particular sodium percarbonate, in a fluidized bed, in such a way that a higher storage stability can be obtained for a given amount of coating than in the case with conventional methods.